Add new config flag for global motion experiment

Change-Id: I312af6af911cd0f52357745324b74e56a8d08d70

configure

diff --git a/configure b/configure
index eda83f1..edb8a36 100755
--- a/configure
+++ b/configure
@@ -278,6 +278,7 @@
     ext_inter
     ext_interp
     ext_refs
+    global_motion
     supertx
     ans
     loop_restoration